This invention relates generally to D/A converting circuits which are used for decoding and converting a digital signal, such as a PCM signal into an analog signal, such as an audio signal, and more particularly, the present invention relates to D/A converting circuit in which expansion is effected so that an input digital signal of given bits can be processed by D/A converters of less bits.
As is well known, the dynamic range of an analog signal processed by an A/D conveter or a D/A converter is determined by the number of entire bits of the A/D converter or the D/A converter.
Since the cost of A/D converters or D/A converters remarkably increases as the bit number increases, in digital devices of low cost for instance, so called compression type A/D converting circuits and expansion type D/A converting circuits are often used so that signal processing can be performed by using low cost A/D converters or D/A converters having less bit numbers where the dynamic range of the signal to be processed is wider than the dynamic range corresponding to the bit number of the converter used.
Although conventional expansion type D/A converting circuits, which will be described hereinafter is capable of ensuring a desired dynamic range by using a D/A converter of a small number of bits by performing expansion of given bits in the case that the input digital signal represents an analog signal magnitude which is greater than one-half of the full-scale value of the analog signal, there is a problem that quantization error occurs greatly on expansion because the subrange or step size on expansion becomes wider than the subrange on nonexpansion. Also the accuracy of resistors used in a variable gain circuit, which is used for expansion, is required to correspond to K bits, while circuit accuracy deteriorates due to D.C. leak in an electronic switch of the variable gain circuit, and therefore, a solution for these has been desired.